Semiconductor processing method of forming a static random access memory cell and static random access memory cell

ABSTRACT

A semiconductor processing method of forming a static random access memory cell having an n-channel access transistor includes, providing a bulk semiconductor substrate; patterning the substrate for definition of field oxide regions and active area regions for the n-channel access transistor; subjecting the patterned substrate to oxidizing conditions to form a pair of field oxide regions and an intervening n-channel access transistor active area therebetween, the field oxide regions having respective bird&#39;s beak regions extending into the n-channel access transistor active area, the n-channel access transistor active area defining a central region away from the bird&#39;s beak regions; and conducting a p-type V T  ion implant into the n-channel active area using the field oxide bird&#39;s beak regions as an implant mask to concentrate the V T  implant in the central region of the active area. A semiconductor device includes, a substrate; an n-type transistor on the substrate; and field oxide surrounding the transistor, the transistor having an active area including a central region and a peripheral region with respect to the field oxide, the transistor having a p-type V T  ion implant which is more concentrated in the central region than in the peripheral region.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of application Ser. No. 08/850,950, filed May 5,1997, now U.S. Pat. No 5,751,096 entitled "A Semiconductor ProcessingMethod of Forming A Static Random Access Memory Cell And Static RandomAccess Memory Cell" listing inventors as Charles H. Dennison and KenMarr, which in turn is a file wrapper continuation of U.S. patentapplication Ser. No. 08/661,824, filed Jun. 11, 1996 now abandoned,which in turn is a division of allowed U.S. patent application Ser. No.08/514,106, filed Aug. 11, 1995, now U.S. Pat. No. 5,650,750 andassigned to the assignee hereof.

TECHNICAL FIELD

This invention relates to non-volatile static memory devices.

BACKGROUND OF THE INVENTION

One known type of static read/write memory cell is a high-density staticrandom access memory (SRAM). A static memory cell is characterized byoperation in one of two mutually-exclusive and self-maintainingoperating states. Each operating state defines one of the two possiblebinary bit values, zero or one. A static memory cell typically has anoutput which reflects the operating state of the memory cell. Such anoutput produces a "high" voltage to indicate a "set" operating state.The memory cell output produces a "low" voltage to indicate a "reset"operating state. A low or reset output voltage usually represents abinary value of zero, while a high or set output voltage represents abinary value of one.

A static memory cell is said to be bistable because it has two stable orself-maintaining operating states, corresponding to two different outputvoltages. Without external stimuli, a static memory cell will operatecontinuously in a single one of its two operating states. It hasinternal feedback to maintain a stable output voltage, corresponding tothe operating state of the memory cell, as long as the memory cellreceives power.

The two possible output voltages produced by a static memory cellcorrespond generally to upper (V_(CC) internal-V_(T)) and low (V_(SS))circuit supply voltages. Intermediate output voltages, between the upper(V_(CC) -V_(T)) and lower (V_(SS)) circuit supply voltages, generally donot occur except for during brief periods of memory cell power-up andduring transitions from one operating state to the other operatingstate.

The operation of a static memory cell is in contrast to other types ofmemory cells such as dynamic cells which do not have stable operatingstates. A dynamic memory cell can be programmed to store a voltage whichrepresents one of two binary values but requires periodic reprogrammingor "refreshing" to maintain this voltage for more than very short timeperiods.

A dynamic memory cell has no internal feedback to maintain a stableoutput voltage. Without refreshing, the output of a dynamic memory cellwill drift toward intermediate or indeterminate voltages resulting inloss of data. Dynamic memory cells are used in spite of this limitationbecause of the significantly greater packaging densities which can beattained. For instance, a dynamic memory cell can be fabricated with asingle MOSFET transistor, rather than the six transistors typicallyrequired in a static memory cell. Because of the significantly differentarchitectural arrangements and functional requirements of static anddynamic memory cells and circuits, static memory design has developedalong generally different paths than has the design of dynamic memories.

Implementing a static memory cell on an integrated circuit involvesconnecting isolated circuit components or devices, such as inverters andaccess transistors, through specific electrical paths. When fabricatingintegrated circuits into a semiconductor substrate, devices within thesubstrate must be electrically isolated from other devices within thesubstrate. The devices are subsequently interconnected to createspecific desired circuit configurations.

One common technique for isolating devices is referred to as LOCOSIsolation (for LOCal Oxidation of Silicon), which involves the formationof a semi-released oxide in the non-active (or field) areas of the bulksubstrate. Such oxide is typically thermally grown by means of wetoxidation of the bulk silicon substrate at temperatures of around 1000°C. for two to six hours. The oxide grows where there is no maskingmaterial over other silicon areas on the substrate. A typical maskingmaterial used to cover areas where field oxide is not desired isnitride, such as Si₃ N₄.

However, at the edges of a nitride mask, some of the oxidant alsodiffuses laterally immediately therebeneath. This causes oxide to growunder and lift the nitride edges. The shape of the oxide at the nitrideedges is that of a slowly tapering oxide wedge that merges into apreviously formed thin layer of pad oxide, and has been termed as a"bird's beak". The bird's beak is generally a lateral extension of thefield oxide into the active areas of devices. Further process steps etchaway part of the bird's beak oxide.

The threshold voltage (V_(T)) of a MOS transistor determines therequirement for turning the MOS transistor on or off. Therefore, it isimportant to be able to adjust the threshold voltage in designing theMOS transistor. One common method of controlling threshold voltage isthrough the use of ion implantation (e.g., boron implantation). Becausevery precise quantities of impurity can be introduced using ionimplantation, it is possible to maintain close control of V_(T). Ashallow boron implant into the p-type substrate of an n-channeltransistor will make the V_(T) more positive with increasing dose.

The threshold voltage V_(T) of a MOS transistor is also affected by gatedimensions. If channel length is long, the influence of the drain andsource junctions on the quantity of charge in the channel is minimal. Onthe other hand, as channel length decreases and approaches thedimensions of the widths of the depletion regions of the source anddrain junctions, the depletion regions become an increasing part of thechannel-depletion region. Some of the channel-depletion region charge islinked to the charge in the depletion region in the source and drainstructures instead of being linked to gate charge. The channel region isdepleted in part without any influence of gate voltage. Therefore,because some of the channel is depleted without a gate bias, less gatecharge is necessary to invert the channel in short channel devices thanin a long channel device of comparable substrate doping.

To be able to establish slightly positive V_(T) values, such as underone volt, in long-channel NMOS transistors with lightly dopedsubstrates, it is necessary to increase the doping concentration at thesurface of the channel which is done by a boron implant, typicallythrough the sacrificial oxide or gate oxide.

There is a parameter in a static memory cell called beta ratio, which isapproximately equal to:

     (pulldownW/pulldownL)/(accessW/accessL)!

where pulldownW is the effective electrical width of the active area ofa pulldown transistor in the static memory cell; where pulldownL is theeffective electrical length of the gate of the pulldown transistor inthe memory cell; where accessW is the effective electrical width of theactive area of an access transistor in the static memory cell; and whereaccessL is the effective electrical length of the gate of the accesstransistor in the static memory cell. This beta ratio is required to beabove a predetermined value, such as 3.0, for stable operation of thestatic memory cell.

It is, of course, desirable to reduce the size of a static memory cell.Typically, to minimize the cell size, the accessW and pulldownL are setat the minimum values as defined by the process capability. Thus, it isnecessary to increase accessL and/or pulldownW to maintain an acceptablebeta ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a circuit schematic of a static memory cell.

FIG. 2 is a circuit schematic of an alternative static memory cell.

FIG. 3 is a diagrammatic top view of a wafer fragment at one processingstep in accordance with the invention.

FIG. 4 is a sectional view taken along line 4--4 of FIG. 3.

FIG. 5 is a view of the FIG. 3 wafer shown at a subsequent processingstep.

FIG. 6 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 5.

FIG. 7 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 6.

FIG. 8 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 7.

FIG. 9 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 8.

FIG. 10 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 9.

FIG. 11 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 10.

FIG. 12 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 11.

FIG. 13 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 12.

FIG. 14 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 13.

FIG. 15 is a view of the FIG. 3 wafer shown at a processing stepsubsequent to that of FIG. 14.

FIG. 16 is a sectional view taken along line 16--16 of FIG. 15.

FIG. 17 is a sectional view taken along line 17--17 of FIG. 15.

FIG. 18 is a sectional view taken along line 18--18 of FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws "to promote the progressof science and useful arts" (Article 1, Section 8).

The invention provides an SRAM cell including an n-type accesstransistor having a reduced effective electrical width (accessW). Thispermits the size of the entire SRAM cell to be reduced, whilemaintaining an acceptable beta ratio. The reduction in effectiveelectrical width of the access transistor is accomplished by performinga p-type V_(T) ion implant into the n-channel active area of the accesstransistor using a field oxide bird's beak as an implant mask toconcentrate the V_(T) implant in a central region of the active area.

In accordance with one aspect of the invention, a semiconductorprocessing method of forming a static random access memory cell havingan n-channel access transistor comprises:

providing a bulk semiconductor substrate;

patterning the substrate for definition of field oxide regions andactive area regions for the n-channel access transistor;

subjecting the patterned substrate to oxidizing conditions to form apair of field oxide regions and an intervening n-channel accesstransistor active area therebetween, the field oxide regions havingrespective bird's beak regions extending into the n-channel accesstransistor active area, the n-channel access transistor active areadefining a central region away from the bird's beak regions; and

conducting a p-type V_(T) ion implant into the n-channel active areausing the field oxide bird's beak regions as an implant mask toconcentrate the V_(T) implant in the central region of the active area.

In accordance with another aspect of the invention, a semiconductorprocessing method of forming a static random access memory cell havingan n-channel access transistor comprises:

providing a bulk semiconductor substrate;

providing a pad oxide layer on the substrate;

patterning the substrate, using a silicon nitride mask, for definitionof field oxide regions and active area regions for the n-channel accesstransistor;

subjecting the patterned substrate to LOCOS oxidizing conditions to forma pair of field oxide regions and an intervening n-channel accesstransistor active area between the field oxide regions, the field oxideregions having respective bird's beak regions extending into then-channel access transistor active area, the n-channel access transistoractive area defining a central region away from the bird's beak regions;

stripping the silicon nitride mask;

patterning photoresist using a mask to regions outside the accesstransistor active area;

conducting a p-type V_(T) ion implant into the n-channel active areausing the field oxide bird's beak regions as an implant mask toconcentrate the V_(T) implant in the central region of the active area;

conducting a germanium implant into the n-channel active area using thefield oxide bird's beak regions as an implant mask to limit diffusion ofthe V_(T) implant;

stripping the photoresist;

stripping the pad oxide;

growing a sacrificial gate oxide;

conducting a p-type V_(T) ion implant into the n-channel active areathrough the sacrificial gate oxide;

stripping the sacrificial gate oxide; and

growing a final gate oxide.

In accordance with another embodiment of the invention, a semiconductordevice comprises:

a substrate;

a static random access memory cell on the substrate, the static randomaccess memory cell including an n-type access transistor; and

field oxide surrounding the access transistor, the access transistorhaving an active area including a central region and a peripheral regionwith respect to the field oxide, the access transistor having a p-typeV_(T) ion implant which is more concentrated in the central region thanin the peripheral region.

In accordance with another embodiment of the invention, a semiconductordevice comprises:

a substrate;

an n-type transistor on the substrate; and

field oxide surrounding the transistor, the transistor having an activearea including a central region and a peripheral region with respect tothe field oxide, the transistor having a p-type V_(T) ion implant whichis more concentrated in the central region than in the peripheralregion.

In accordance with another embodiment of the invention, a semiconductorprocessing method of forming a transistor comprises:

providing a bulk semiconductor substrate;

patterning the substrate for definition of field oxide regions andactive area regions for the transistor;

subjecting the patterned substrate to oxidizing conditions to form apair of field oxide regions and an intervening transistor active areatherebetween, the field oxide regions having respective bird's beakregions extending into the transistor active area, the transistor activearea defining a central region away from the bird's beak regions; and

conducting an ion implant into the transistor active area using thefield oxide bird's beak regions as an implant mask to concentrate theimplant in the central region of the active area.

A static memory cell 10 is illustrated in FIG. 1 which embodies theinvention. Static memory cell 10 generally comprises first and secondinverters 12 and 14 which are cross-coupled to form a bistableflip-flop. Inverters 12 and 14 are formed by n-channel pulldow (driver)transistors 16 and 17, and p-channel load (pullup) transistors 18 and19. Transistors 16 and 17 are typically metal oxide silicon field effecttransistors (MOSFETS) formed in an underlying silicon semiconductorsubstrate. P-channel transistors 18 and 19 can be thin film transistorsformed above the driver transistors or bulk devices.

The source regions of driver transistors 16 and 17 are tied to a lowreference or circuit supply voltage, labelled V_(SS), and typicallyreferred to as "ground." Load transistors 18 and 19 are connected inseries between a high reference or circuit supply voltage, labelledV_(CC), and the drains of the corresponding driver transistors 16 and17. The gates of load transistors 18 and 19 are connected to the gatesof the corresponding driver transistors 16 and 17.

Inverter 12 has an inverter output 20 formed by the drain of drivertransistor 16. Similarly, inverter 14 has an inverter output 22 formedby the drain of driver transistor 17. Inverter 12 has an inverter input24 formed by the gate of driver transistor 16. Inverter 14 has aninverter input 26 formed by the gate of driver transistor 17.

The inputs and outputs of inverters 12 and 14 are cross-coupled to forma flip-flop having a pair of complementary two-state outputs.Specifically, inverter output 20 is cross-coupled to inverter input 26,and inverter output 22 is cross-coupled to inverter input 24. In thisconfiguration, inverter outputs 20 and 22 form the complementarytwo-state outputs of the flip-flop.

A memory flip-flop such as that described typically forms one memoryelement of an integrated array of static memory elements. A plurality ofaccess transistors, such as access transistors 30 and 32, are used toselectively address and access individual memory elements within thearray. Access transistor 30 has one active terminal connected tocross-coupled inverter output 20. Access transistor 32 has one activeterminal connected to cross-coupled inverter output 22. A pair ofcomplementary column lines 34 and 36 shown, are connected to theremaining active terminals of access transistors 30 and 32,respectively. A row line 38 is connected to the gates of accesstransistors 30 and 32.

Reading static memory cell 10 requires activating row line 38 to connectinverter outputs 20 and 22 to column lines 34 and 36. Writing to staticmemory cell 10 requires complementary logic voltage on column lines 34and 36 with row line 38 activated. This forces the outputs to theselected logic voltages, which will be maintained as long as power issupplied to the memory cell, or until the memory cell is reprogrammed.

At least one of the access transistors 30 and 32 is formed using themethod described below starting with the description of FIG. 3.

Shown in FIG. 2 is an alternative static memory cell 50 embodying theinvention. Static memory cell 50 comprises n-channel pulldown (driver)transistors 80 and 82 having drains respectfully connected to loadelements or resistors 84 and 86. Transistors 80 and 82 are typicallymetal oxide silicon field effect transistors (MOSFETs) formed in anunderlying silicon semiconductor substrate.

The source regions of transistors 80 and 82 are tied to a low referenceor circuit supply voltage, labelled V_(SS) and typically referred to as"ground." Resistors 84 and 86 are respectively connected in seriesbetween a high reference or circuit supply voltage, labelled V_(CC), andthe drains of the corresponding transistors 80 and 82. The drain oftransistor 82 is connected to the gate of transistor 80 by line 76, andthe drain of transistor 80 is connected to the gate of transistor 82 byline 74 to form a flip-flop having a pair of complementary two-stateoutputs.

A four transistor memory element, such as that described above inconnection with FIG. 2, typically forms one memory element of anintegrated array of static memory elements. A plurality of accesstransistors, such as access transistor 90 and 92, are used toselectively address and access individual memory elements within thearray. Access transistor 90 has one active terminal connected to thedrain of transistor 80. Access transistor 92 has one active terminalconnected to the drain of transistor 82. A pair of complementary columnlines 52 and 54 shown, are connected to the remaining active terminalsof access transistors 90 and 92, respectively. A row line 56 isconnected to the gate of access transistors 90 and 92.

Reading static memory cell 50 requires activating row line 56 to connectoutputs 68 and 72 to column lines 52 and 54. Writing to static memorycell 10 requires complementary logic voltages on column lines 52 and 54with row line 56 activated. This forces the outputs to the selectedlogic voltages, which will be maintained as long as power is supplied tothe memory cell, or until the memory cell is reprogrammed.

At least one of the access transistors 90 and 92 is formed using themethod described below.

A semiconductor processing method of forming a static random accessmemory cell is shown in FIGS. 3-16. The method comprises forming ann-channel access transistor, by performing the following steps.

As illustrated in FIG. 4, a bulk semiconductor substrate 100 isprovided. In the illustrated embodiment, the substrate is a p-substrate.The substrate 100 is cleaned, and a pad oxide layer 102 is thermallygrown or deposited on the substrate by CVD (Chemical Vapor Deposition).In the illustrated embodiment, the pad oxide layer 102 comprises 10-40nm of SiO₂. The purpose of the pad oxide layer 102 is to cushion thetransition of stresses between the substrate and subsequently depositedmaterial during field oxidation.

The substrate 100 is then patterned for definition of field oxideregions and active area regions for the n-channel access transistor.More particularly, masking material 106 is used to cover areas wherefield oxide is not desired. In the illustrated embodiment, the maskingmaterial 106 is a 100-200 nm thick layer of CVD silicon nitride. Siliconnitride is selected as a masking material because oxygen and water vapordiffuse very slowly through it, thus preventing oxidizing species fromreaching the silicon surface under the nitride.

The patterned substrate is then subjected to oxidizing conditions toform a pair of field oxide regions 110 and 112, and an interveningn-channel access transistor active area 114 between the field oxideregions 110 and 112. More particularly, the substrate is subjected toLOCOS isolation. LOCOS isolation (LOCal Oxidation of Silicon) involvesthe formation of a semi-recessed oxide in the non-active (or field)areas of the bulk substrate. The oxide is thermally grown by means ofwet oxidation of the bulk silicon substrate at temperatures of around1000° C. for two to six hours. The oxide grows where there is no maskingmaterial over other silicon areas on the substrate. The LOCOS isolationresults in the formation of peripheral bird's beak regions 118 and 120,which extend into the n-channel access transistor active area 114. Then-channel access transistor active areas 114 includes a central region115, away from the bird's beak regions 118 and 120, and a peripheralregion 116 outside the central region 115 (FIG. 3).

After LOCOS isolation is performed, the masking material 106 is stripped(FIG. 5). Photoresist mask 122 is then non-critically applied to maskregions 124 and 126 outside the n-channel access transistor active area114 (FIG. 6) against a V_(T) ion implant which is described below.

After the photoresist is applied, a p-type (e.g., boron) V_(T) ionimplantation into the n-channel active area is performed (FIG. 7). TheV_(T) ion implantation is performed under conditions which enable usingthe thicker field oxide bird's beak regions as an implant mask, todefine a V_(T) implant 128. This concentrates the V_(T) implant 128 inthe central region of the active area 114, and away from peripheralregion 116 (FIG. 3). Implant energy is selected such that ions do notpenetrate the exposed field oxide of the bird's beak regions into thesubstrate.

An example impurity for the V_(T) implant is boron (B₁₁). The boronimplantation is preferably performed at low energy; e.g., at between 2and 8 KeV. The dose of the implanted boron impurity is between 2×10¹¹and 1×10¹³ ions per square centimeter. After implant, the volumetricconcentration near the surface of the implanted impurity 128 isapproximately between 1×10¹⁵ and 1'10¹⁷ ions per cubic centimeter.

An alternative example impurity for the V_(T) implant is BF₂. The BF₂implantation is preferably performed at low energy; e.g., at between 10and 30 KeV. Because BF₂ is a larger molecule than B₁₁, more energy canbe used to obtain a depth comparable to the depth of B₁₁ when B₁₁ isimplanted. If the same energy were used as would be used for B₁₁, ashallower implant would result. Using some commercial implanters, a highenergy BF₂ implant takes less time than a low energy B₁₁ implant. Thedose of the implanted BF₂ impurity is between 2×10¹¹ and 1×10¹³ ions persquare centimeter. After implant, the volumetric concentration near thesurface of the implanted BF₂ impurity is approximately between 1×10¹⁵and 1×10¹⁷ ions per cubic centimeter.

The ion implantation is preferably performed before performing any stepsthat etch away at the bird's beak regions.

After the V_(T) implant is performed, an ion implantation is preferablyperformed using a material which impedes diffusion of the V_(T) implant.More particularly, in the illustrated embodiment, a germanium ionimplantation into the n-channel active area is performed (FIG. 8) todefine a germanium implant defined in the figure as a boundary line 130.The germanium implant 130 impedes diffusion of the V_(T) implant. Thegermanium implantation is also preferably performed using the fieldoxide bird's beak regions as an implant mask. The use of germaniumimplants to retard boron diffusion is described in an article titled"Novel Germanium/Boron Channel-Stop Implantation for Submicron CMOS", byJames R. Pfiester and John R. Alvis, 1987, IEEE IEDM Digest, pp.740-741, which is incorporated herein by reference.

In the illustrated embodiment of the invention, the germaniumimplantation is performed at between 50 and 200 KeV. In the illustratedembodiment, the dose of the implanted germanium is between 5×10¹³ and1×10¹⁵ ions per square centimeter. In the illustrated embodiment, thevolumetric concentration of the implanted germanium near the surface isapproximately between 1×10¹⁷ and 5×10²⁰ ions per cubic centimeter.

After the germanium implant is performed, the photoresist (used to maskregions 124 and 126) is stripped (FIG. 9).

After the photoresist is stripped, the pad oxide 102 is stripped (FIG.10). This results in the bird's beak regions being stripped back.

After the pad oxide is stripped, a sacrificial oxide 132 is grown (FIG.11) in view of the Kooi effect. Kooi et al. discovered that NH₃ isgenerated from a reaction between H₂ O and masking nitride during fieldoxidation at the pad-oxide/silicon interface. This NH₃ diffuses throughthe pad oxide and reacts with the silicon substrate to formsilicon-nitride spots, which impede the growth of gate oxide and causelow voltage breakdown of the gate oxide. Damage or unwanted nitride isremoved by growing (FIG. 11) and later removing the sacrificial oxide132.

After the sacrificial oxide 132 is grown, a conventional unmaskedenhancement implant is performed (FIG. 12). This is another preferredthreshold voltage adjust implant (V_(T) implant), preferably of boron.The implant is an unmasked implant into all transistors on thesubstrate. The implant can be performed through the oxide 132. Thisboron implantation is preferably performed at an energy level between 20and 80 KeV. The dose of the implanted boron impurity is between 1×10¹²and 1×10¹³ ions per square centimeter. Conventional masked implants,such as a masked depletion implant, may then be performed.

After the enhancement implant and other masked implants are performed,the sacrificial gate 132 is stripped (FIG. 13), resulting in the bird'sbeak regions being further stripped back.

After the sacrificial oxide is stripped, a final gate oxide 134 is grown(FIG. 14) in the exposed active area 114. In the illustrated embodiment,the final gate oxide 134 is grown through dry oxidation in a chlorineambient.

FIG. 15 is a top view of the wafer after the source and drain regionsare formed, and other conventional steps are completed. The completedaccess transistor is indicated by reference numeral 200 (FIGS. 16-17).Note that because the bird's beak regions have been stripped back byvarious process steps, central region 115 has a greater V_(T)concentration then the completed peripheral region 116. FIG. 15 showsrow line 202 which can act as row line 38 shown in FIG. 1, or row line56 shown in FIG. 2.

FIG. 16 is a sectional view taken along line 16--16 of FIG. 15. The rowline 202 comprises the gate oxide 134, conductively doped polysiliconregion 144, an overlying WSi_(X) layer 152, and an overlying oxide layer156 (FIG. 16). Oxide insulating sidewall spacers 158 are also providedrelative to the illustrated row line. The completed access transistor200 further includes source and drain regions 148 and 150, and LDDregions 160.

FIG. 17 is a sectional view taken along line 17--17 of FIG. 15 andillustrates that high V_(T) does not exist in the peripheral region 116.

FIG. 18 is a sectional view taken along line 18--18 of FIG. 15 and showsthe centralized location of the V_(T) implant 128.

Thus, an SRAM cell is provided including an n-type access transistorhaving a reduced effective electrical width. The size of the entire SRAMcell is reduced, while an acceptable beta ratio is maintained. Thereduction in effective electrical width of the access transistor isaccomplished by performing a p-type V_(T) ion implant into the n-channelactive area of the access transistor using a field oxide bird's beak asan implant mask to concentrate the V_(T) implant in a central region ofthe active area.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

We claim:
 1. A semiconductor device comprising:a substrate; a staticrandom access memory cell on the substrate, the static random accessmemory cell including an n-type access transistor; field oxidesurrounding the access transistor, the access transistor having acentral region, a bird's beak area surrounding the central region, afield oxide area connected to and extending from the bird's beak area,and an area below the bird's beak, the central region having a p-typeV_(T) ion implant concentration, the area below the bird's beak beingsubstantially void of the V_(T) ion implant; and a germanium implant inthe central region.
 2. A semiconductor device in accordance with claim 1wherein the V_(T) ion implant comprises B₁₁.
 3. A semiconductor devicein accordance with claim 1 wherein the V_(T) ion implant comprises BF₂.4. A semiconductor device in accordance with claim 1 wherein the implantis defined by performing an implantation at an energy between 10 and 30KeV.
 5. A semiconductor device in accordance with claim 1 and furthercomprising a germanium implant in the n-channel active area.
 6. Asemiconductor device comprising:a substrate; a static random accessmemory cell on the substrate, the static random access memory cellincluding an n-type access transistor; and field oxide surrounding theaccess transistor, the access transistor having a central region, abird's beak area surrounding the central region, a field oxide areaconnected to and extending from the bird's beak area, and an area belowthe bird's beak, the central region having a BF₂ V_(T) ion implant witha concentration resulting from an implantation of BF₂ at an energybetween 10 and 30 KeV and a dose between 2×10¹¹ and 1×10¹³ ions persquare centimeter, using the bird's beak as an implant mask beforeprocessing that etches away at the bird's beak, the area below thebird's beak having a V_(T) implant concentration that is substantiallylower than the V_(T) ion implant concentration of the central region. 7.A semiconductor device in accordance with claim 6 and further comprisinga germanium implant in the central region.
 8. A semiconductor devicecomprising:a substrate; an n-type transistor formed on the substrate;field oxide surrounding the transistor, the transistor having a centralregion, a bird's beak area surrounding the central region, a field oxidearea connected to and extending from the bird's beak area, and an areabelow the bird's beak, the central region having a p-type V_(T) ionimplant concentration, the area below the bird's beak having a V_(T)implant concentration that is substantially lower than the V_(T) ionimplant concentration of the central region; and a diffusion retardinggermanium implant in the central region.
 9. A semiconductor device inaccordance with claim 8 wherein the V_(T) ion implant comprises B₁₁. 10.A semiconductor device in accordance with claim 8 wherein the V_(T) ionimplant comprises BF₂.
 11. A semiconductor device comprising:asubstrate; a static random access memory cell on the substrate, thestatic random access memory cell including an n-type access transistor;and field oxide surrounding the access transistor, the access transistorhaving a central region, a bird's beak area surrounding the centralregion, a field oxide area connected to and extending from the bird'sbeak area, and an area below the bird's beak, the central region havinga p-type V_(T) ion implant concentration, the area below the bird's beakhaving a V_(T) ion implant concentration that is substantially lowerthan the V_(T) ion implant concentration of the central region.
 12. Asemiconductor device in accordance with claim 11 wherein the V_(T) ionimplant comprises B₁₁.
 13. A semiconductor device in accordance withclaim 11 wherein the V_(T) ion implant comprises BF₂.
 14. Asemiconductor device in accordance with claim 13 wherein the implant isdefined by performing an implantation at an energy between 10 and 30KeV.
 15. A semiconductor device in accordance with claim 11 and furthercomprising a germanium implant in the n-channel active area.
 16. Asemiconductor device comprising:a substrate; a static random accessmemory cell on the substrate, the static random access memory cellincluding an n-type access transistor; and field oxide surrounding theaccess transistor, the access transistor having a central region, abird's beak area surrounding the central region, a field oxide areaconnected to and extending from the bird's beak area, and an area belowthe bird's beak, the central region having a BF₂ V_(T) ion implant witha concentration resulting from an implantation of BF₂ at an energybetween 10 and 30 KeV and a dose between 2×10¹¹ and 1×10¹³ ions persquare centimeter, the area below the bird's beak having a V_(T) implantconcentration that is substantially lower than the V_(T) ion implantconcentration of the central region.
 17. A semiconductor device inaccordance with claim 16 and further comprising a germanium implant inthe central region.
 18. A semiconductor device comprising:a substrate;an n-type transistor formed on the substrate; field oxide surroundingthe transistor, the transistor having a central region, a bird's beakarea surrounding the central region, a field oxide area connected to andextending from the bird's beak area, and an area below the bird's beak,the central region having a p-type V_(T) ion implant concentration, thearea below the bird's beak having a V_(T) implant concentration that issubstantially lower than the V_(T) ion implant concentration of thecentral region; and a germanium implant in the central region.
 19. Asemiconductor device in accordance with claim 18 wherein the V_(T) ionimplant comprises B₁₁.
 20. A semiconductor device in accordance withclaim 18 wherein the V_(T) ion implant comprises BF₂.